Silicon molecular beam epitaxy (MBE) is a technique for low temperature growth of single crystal silicon epitaxial layers in an ultra-high vacuum station. One system that has been used for silicon MBE processing is disclosed by Y. Ota in the article entitled "Silicon Molecular Beam Epitaxy with Simultaneous Ion Implant Doping," Journal of Applied Physics, Vol. 51 (2), February 1980, pp. 1102-1110. In the Ota system, a high purity source of silicon is evaporated by an electron beam and the evaporated silicon deposits as single crystal epitaxy on a heated silicon substrate. In order to create devices that are useful, the silicon epitaxy is usually doped with p-type or n-type impurities. In the Ota system, an ion gun generates ions of the required dopant and this dopant is coupled through a drift chamber to the growth chamber in which the silicon deposition takes place. As a result, very low energy dopant ion implantation is coincident with the process of growing an epitaxial layer. By using ion implant doping the number of stray dopant atoms is reduced from other prior art techniques wherein the dopant was simply evaporated in order to deposit on the growing substrate. Fewer dopant atoms are introduced into the growth chamber because the sticking coefficient of the implanted dopant is much higher than that of an evaporated dopant. An additional advantage in ion implant doping is that the dopant ion beam density can be monitored and controlled during the epitaxial growth.
In the system disclosed by Ota, arsenic ions from an arsenic plasma are developed by the ion gun and propelled through the drift chamber to the growing epitaxial substrate thereby establishing an n-type dopant in the silicon substrate. There is no suggestion in the Ota article as to how one might establish a p-type dopant in the growing epitaxial layers. One technique that would be apparent to those skilled in the art is to simply terminate growth of the epitaxial structure while a different plasma consisting of a p-type dopant is established in the ion gun. It is desirable that growth should be suspended for a minimal length of time when converting from one dopant source to another since when growth is suspended, ambient oxygen, carbon and other contaminants accumulate on the sample surface thereby providing a source of crystalline disruption for any subsequent epitaxial growth. These defects in the crystal structure are known to impair the operation of the semiconductor device and also to result in a decreased minority-carrier lifetime. It is therefore desirable to change from one type of conductivity to another in as short a period as possible with as little disruption in growth as possible.